The automotive industry utilizes belt tensioners to create a generally constant tension on a belt as it changes in length from normal wear, or from changes in span lengths due to span speed differences, wherein the belt is driven by a single drive pulley from the output shaft of the engine and wherein the belt rotates driven pulleys, each operating an automotive accessory. In known designs, the belt tensioner includes either a flat-wound spring or a helical spring, a spring casing, and an arm. One end of the spring is attached to the spring casing and the other end of the spring is attached to the arm. The arm pivots with respect to the spring casing when the spring exerts torque. The spring casing is attached to the engine, and an idler pulley is attached to the arm. The spring is preloaded by twisting the arm relative to the spring casing. The idler pulley on the arm is then placed against the belt. As the belt span lengthens, the torque from the preloaded spring continues to cause the idler pulley of the arm to apply pressure against the belt keeping the belt in tension.
In one known arrangement patented as U.S. Pat. No. 5,772,549, a helical spring has a first end screwed into a first screw-like passage of the arm of the spring tensioner and has a second end screwed into a second screw-like passage of the spring casing. The spring is under tension and holds the parts together while permitting the arm to rotate relative to the spring casing. A cone-shaped bushing is disposed inside the spring between a portion of the arm and a portion of the spring casing to facilitate the rotation of the arm relative to the spring casing. This arrangement is open to contamination and the spring configuration creates a moment loading.
In one known example of a belt tensioner, a square-shaped hole in the idler pulley is engaged by a square head, often as would be common to a ½″ or ⅜″ leveraging or ratchet or similar wrench, to lift (preload) the arm. In a different known example, the idler pulley is mounted on a post of the arm, wherein the post has an annular rim which is deformed radially outward and over the bearing of the idler pulley creating a radial rivet joint to retain the idler pulley to the arm.
In one known method, the arm is cast using a mold having first and second sections, and the spring case is cast using a mold having first and second segments. In this method, a path in the belt tensioner from the arm's seat for the bearing of the idler pulley to the engine mounting surface of the spring case crosses a line on the spring case corresponding to the parting line of the first and second segments.
Still, engineers continue to seek improved belt tensioners.